Telecommunication networks can be divided into circuit switched networks and packet switched networks. In circuit switched networks, the communication is allocated a circuit prior to the beginning of the transmission. An example of a circuit allocated to users A and B is given in FIG. 1 showing the allocated circuit that can only be used by these users. Information about the recipient of the sent information is readily included in the circuit identity. The main disadvantage of this switching method is that the circuit is reserved even though there is no information to be sent.
In connectionless packet switched networks, the transmission media is common to all users. The information is sent in packets, and all packets contain information about their destination. There is no need to allocate transmission resources for the communication prior to the beginning of the transmission. No packets are transmitted if there is no information to be sent. Thus, the network capacity is not reserved in vain. Based on the information about the destination included in the packet, every network element routes the packet to the next network element. The possible routes for the packets sent by user A to user B are shown in FIG. 2. Basically, all the packets sent from terminal A to terminal B do not necessarily travel through the same route.
In connection oriented packet switched techniques, a method of establishing virtual circuits is known. A virtual circuit comprises predetermined legs between network elements, and every packet in a connection is routed along the same route. Thus, the information is routed as in circuit switched networks shown in FIG. 1, but the communication capacity is not reserved (in vain) if there is nothing to be sent. Every packet includes information about its virtual circuit, and every network element holds context information which tells where to route a packet with a known virtual circuit to and what identifiers to use on the next leg. An example of a technique utilising virtual circuits is the well known ATM (Asynchronous Transfer Mode) technology.
It is also known that the methods of virtual circuit and connectionless packet switching with no virtual circuits can be combined. In such a method, there are some network elements via which all the packets are routed. An example of such a method is given in FIG. 3. In FIG. 3, a virtual circuit passing through network elements 12 and 22 is allocated between the sender A and network element 32. Network element 32 holds context information for the connection, and knows that the packets on that virtual circuit are destined to receiver B, which is connected to network element 53. Between network elements 32 and 52, a connectionless packet switched network is used, and packets from element 32 can be routed to element 52 along different paths. Though, all the packets are routed through network elements 12, 22, 32 and 53, which thus compose a virtual circuit between terminals A and B. It is worth noting that A would not be able to establish a connection with B if e.g. network element 32 would not hold the necessary context information concerning the connection. That information is not held by e.g. network element 31. Therefore, all the data packets of the connection have to be routed via network element 32 as well as through elements A, 12, 22, 53 and B, which are the key network elements of the connection.
An example of a system utilising virtual circuits is the General Packet Radio Service GPRS being specified by ETSI (European Telecommunications Standards Institute). The basic structure of the GPRS network is shown in FIG. 4. The elements shown are Serving GPRS Support Node (SGSN1, SGSN2), Gateway GPRS Support Node (GGSN1, GGSN2) and the BSS (Base Station Subsystem) consisting of a Base Station Controller (BSC1, BSC2) and many Base Transceiver Stations (BTS11, BTS12, BTS21, BTS22). Connections to other networks (not shown), such as Internet or an X.25 network, are made via the GGSN. Additionally, the network includes a Home Location Register (HLR) where e.g. information about the subscribed services is kept.
Basically, when a mobile station MS is located in a cell, every packet destined to or sent by mobile station MS is transmitted through the same BTS, same BSC, same SGSN and same GGSN. The MS cannot establish a connection to the GGSN if the used SGSN does not hold context information for this MS. The mobile MS is located in cell CELL11 and communicating with a BTS, BTS11, through the radio interface Um. Between the BTS and the SGSN, a virtual circuit is established, and all the packets are transmitted along the same route. In the connectionless packet switched network using the Internet Protocol (IP) between the SGSN and the GGSN, the transmission of different packets may use different routes.
The link between the mobile MS and the SGSN is uniquely identified by routing area RA and the Temporary Logical Link Identity TLLI. Routing area consists of one or several cells, and is used in the GPRS mobility management as location information for mobiles in a so-called standby state in which the mobile has no active connections. The TLLI identifies the connection unambiguously within one routing area. A mobile can have multiple simultaneous connections using different protocols, e.g. X.25 and IP. Connections using different protocols are discriminated using a Network Layer Service Access Point Identity NSAPI.
The application layer in the MS sends the SNDCP layer a PDP PDU (Packet Data Protocol Packet Data Unit) which can be, e.g., an IP packet. In the SNDCP layer, the PDU is encapsulated in an SNDCP packet in the header of which the NSAPI is indicated, and the resulting SNDCP packet is sent to LLC layer. The link layer identity TLLI is included in the LLC header. The LLC frames are carried over the air interface Um by the RLC (Radio Link Control) protocol and between the BSC and SGSN by the BSSGP (Base Station Subsystem GPRS Protocol). For downlink packets, the BSS checks the cell identity indicated in the BSSGP header, and routes the cells to the appropriate BTS. For the uplink packets, the BSC includes the BSSGP header the cell identity of the mobile MS based on the source BTS.
Between SSGN and GGSN, the link is identified by the SGSN and GGSN addresses and tunnel identifier TID which identifies the connection in the GGSN and in the SGSN. On the link between the SGSN and the GGSN, the GTP (GPRS Tunnelling Protocol) is used.
GPRS is a system where a kind of a virtual connection is used between MS and GGSN. This connection consists of two separate links, the MS-SGSN link and the SGSN-GGSN link. The MS and the GGSN are not able to communicate with each other if they are not using an SGSN holding the context information for this MS. Therefore, the SGSN in a key network element.
Routing of packets in the GPRS network is presented in the signalling chart of FIG. 5. In the figure, routing of both mobile originated (MO) and mobile terminated (MT) packets is shown. The routing of MO packets is studied first. The MS sends the BSS a data packet containing the TLLI, NSAPI and the user data. On the link between MS and SGSN, the SNDCP (Subnetwork Dependent Convergence Protocol) protocol on LLC (Logical Link Control) protocol is used. In a simple implementation, one BSC is always using the same SGSN, and therefore its function is to route the packets between many BTS's and one SGSN. In a more complicated implementation, the BSC is connected to a plurality of SGSN's and its routing function is also using the TLLI. In such implementation the key network elements of the connection are the MS, the BTS, the BSC, the SGSN and the GGSN that all hold context information necessary to route the packets belonging to the connection. In the BSS this information is stored in a look-up table. An example of a possible look-up table is shown the following:
source TLLI destination SGSN1 11 CELL11 SGSN1 12 CELL12 SGSN2 21 CELL11 SGSN2 22 CELL12 CELL11 11 SGSN1 CELL11 21 SGSN2 CELL12 12 SGSN1 CELL12 22 SGSN2
According to the look-up table above, e.g. packets with TLLI=11 received from SGSN1 are forwarded to BTS11 for transmission over the air interface Um in cell CELL11.
Each SGSN holds context information about each mobile it handles. In GPRS, the context information can be divided into mobility management (MM) and the packet data protocol (PDP) context part. Basically, the mobility management part tells where the mobile is located and in which state (idle, standby, ready) it is, and is common for all the different packet data services using different protocols. The packet data protocol part tells information specific for the service in question, and includes, e.g. routing information and PDP (packet data protocol) address used. Based on the context information, the SGSN maps the identifications TLLI and NSAPI used in the link between the SGSN and the MS to GGSN address and tunnel identifier TID, which identifies the connection between the SGSN and the GGSN. The GGSN then sends the data packet PDP PDU (PDP=Packet Data Protocol PDU=Protocol Data Unit) to the external packet data network.
For mobile terminated packets, the GGSN receives a packet sent to the MS from the external Packet Data Network (PDN). The GGSN knows which SGSN handles the connections of the MS and the identifier TID which identifies the connection in the SGSN. The packet is sent to the SGSN handling the MS, and the SGSN derives from the TID the TLLI, the NSAPI, the routing area identification RAI and eventually if not readily the cell identity CELLID. Based on this, the SGSN can send the packet to the right BSS. Using the TLLI, routing area and cell identity, the BSS can transfer the packet to the right MS. NSAPI is needed in the MS in order to be able to discriminate between different packet data protocols.
The problem of the above described routing method is its rigidity when a key network element in the connection has to be modified. In the case of GPRS, SGSN is the key network element in the connection between the MS and the GGSN. It is normally changed only if the MS moves to the coverage area of another SGSN, which is known as an inter-SGSN routing area update. A need for changing a key network element in the connection can arise for many other reasons, such as when a network element breaks down, a new network element is installed, when a network element has to be shut down for operation and maintenance reasons, or when the traffic load in a network element is too high. In prior art, the change of a key network element cannot be done without interrupting all the ongoing connections.
In GPRS, to change the SGSN via which the traffic from and to a routing area is routed, the network must be reconfigured and all the connections from mobiles in that routing area to the gateway GPRS support nodes GGSN must re-established. In practice the MS's have to reattach to GPRS and reactivate their PDP contexts. This causes unnecessary load in all network elements and in the transmission network, especially in the limited communication bandwidth on the radio interface Um.
The objective of this invention is to solve or at least relieve these problems. This objective is achieved by using a solution according to the invention which is defined in claim 1.